A. Technical Field
The present invention relates to: a coating composition for forming a coating film that can give high scratch resistance to, for example, parts for car exteriors; and a multilayered coating film forming method and a coated article using this coating composition.
B. Background Art
Plastic moldings manufactured from, for example, poly(methyl methacrylate) resin, polymethacrylimide resin, polycarbonate resin, poly(ethylene terephthalate) resin, poly(butylene phthalate) resin, polystyrene resin, AS resin, ABS resin, polyolefin resin and the like are lightweight and good in impact resistance, and widely used as such as plastic materials for cars. However, because the surfaces of these plastic moldings do not have sufficient scratch resistance, these plastic moldings have such faults that: they easily undergo damages on their surfaces by contact and friction with other hard things, scratching, and the like, and the damages done to the surfaces deteriorate their commodity values remarkably, or make their commodities unusable in a short term. Thus, as a means to give scratch resistance to plastic moldings, it is usually carried out to coat the surfaces with a coating to form coating films for enhancing the scratch resistance. Therefore, especially in car uses, the demand of enhancing the scratch resistance of a coating film is high. The demand of enhancing the scratch resistance of a coating film is not limited to plastic materials, but similarly exists also as to the coating film on parts where a metallic material is used, for example, such as bodies.
By the way, a coating film to be formed on a plastic molding is desired to be a coating film that contributes also to the improvement of the capabilities such as weather resistance and stain resistance and must have sufficient adhesion. As a coating film which meets such requirements, a coating film from an isocyanate-curing acrylic polyol coating (an acryl-isocyanate-based coating film) has so far been thought to be suitable in terms of being curable at low temperatures, being excellent in adhesion, being good in various capabilities such as weather resistance and stain resistance, and also being advantageous in the cost. Although the coating film formed for enhancing the scratch resistance of a plastic molding is also desired to be the acryl-isocyanate-based coating film having such advantages as describe above, the acryl-isocyanate-based coating film is generally not high in scratch resistance. Consequently, a technique to enhance the scratch resistance of the acryl-isocyanate-based coating film is demanded.
Thus, as a coating being able to form an acryl-isocyanate-based coating film that has high scratch resistance, so far the following compositions have been proposed: a clear coating composition such that a fluorine-containing copolymer having a hydroxyl value within a specific range, an acrylic copolymer having a hydroxyl value within a specific range, and an isocyanate prepolymer as a curing agent are combined at a specific ratio (see patent document 1); a photocuring resin composition containing an urethane compound having a specific structure, a photopolymerizable acrylic monomer having a specific structure and a photopolymerization initiator (see patent document 2); and the like.
On the other hand, as a means to enhance the scratch resistance by coating other than forming the acryl-isocyanate-based coating film, there have been proposed techniques of attempting to enhance the scratch resistance by introducing a polycaprolactone into a polysiloxane-based coating film and thereby enhancing the elasticity. In concrete terms, the following coating compositions have been proposed: a coating composition containing a polydimethylsiloxane-based copolymer, polycaprolactone and polysiloxane; a coating composition containing a dimethylsiloxane-based copolymer having a backbone into which polycaprolactone and polysiloxane have been introduced; a coating composition containing a polydimethylsiloxane-based copolymer with a backbone into which polycaprolactone has been introduced and further containing polysiloxane; and a coating composition containing a polydimethylsiloxane-based copolymer with a backbone into which polysiloxane has been introduced and further containing polycaprolactone (see patent documents 3 and 4).
[Patent Document 1] JP-A-05-032935 (Kokai)
[Patent Document 2] JP-A-2000-297112 (Kokai)
[Patent Document 3] JP-A-11-228905 (Kokai)
[Patent Document 4] JP-A-2001-011376 (Kokai)
However, though in both the above-mentioned conventional acryl-isocyanate-based coating film and polysiloxane-based coating film, a certain degree of scratch resistance is obtained, yet, for example, in uses of parts for car exteriors and others, when the surface of a coating film is rubbed under strong external force by a cloud of sand flying and others while car washing and running, such problems occurred occasionally that the inside of the coating film went so far as being shaved off in addition to the damage of the surface of the coating film. For details, the coating composition described in the patent document 1 is aimed at increasing the hardness and improving the scratch resistance, as a result, by introducing an acrylic copolymer, and the photocuring resin composition described in the patent document 2 is aimed at increasing the hardness and improving the scratch resistance, as a result, by increasing the crosslinking density through the introduction of a structure for promoting the photopolymerization by ultraviolet irradiation. However, when the crosslinking density is too high, such problems as to decrease adhesion or to induce cracks are caused by the increase of the strain when being cured and shrunk. From such a reason, there is a limit in raising the hardness in any case, and consequently, it was difficult to achieve such a degree of hardness as to be durable (without the inside of the coating film being shaved off) when the surface of the coating film is rubbed under strong external force as described above. Also, the coating compositions described in the patent documents 3 and 4 are aimed at, by the introduction of the polycaprolactone to provide the elasticity, reducing the impact and improving the scratch resistance, as a result, when a cloud of sand and the like contact with the coating film. However, the hardness is insufficient (for example, it is clearly lower even when compared with the acryl-isocyanate-based coating film sufficiently cured). Therefore, when the surface of the coating film was rubbed under such strong external power as described above, still the coating film was occasionally shaved off to the inside.